1. Field of the Invention
The present invention relates to a method of forming a wiring pattern on panel substrates of display devices such as liquid crystal display devices and plasma displays. Specifically, the present invention relates to a method of forming a wiring pattern using inks containing metallic nanoparticles.
2. Description of the Related Art
Conventionally, various processes, that is, a sputtering process, a photolithography process and an etching process are used to form a wiring pattern on substrates that are used for liquid crystal display devices and the like and include thin film transistors (TFTs) arranged in an array.
For example, the sputtering process is the process for adhering a metal film or the like, constituting a wiring pattern, to the substrate, and the photolithography process is the process for forming a predetermined pattern for a wiring pattern. In addition, the etching process is the process for removing unnecessary portions while leaving necessary portions that will constitute the wiring pattern.
When TFTs are intended to be formed on the substrate in this way, the manufacturing cost is increased due to the use of vacuum systems in the sputtering process. As an alternative to the above-described method of forming a wiring pattern, the technique disclosed in Japanese Laid-Open No. 2001-35814 (hereinafter referred to as patent document 1) is known, which is the method of applying a metallic nanoparticles-containing ink onto a substrate. The metal nanoparticles are conductive ultrafine particles (hereinafter referred to as nanoparticles) with particle diameters of the order of nanometer (1 nm=0.001 μm).
FIGS. 1A to 1E are cross-sectional views of principal portions of a substrate, which are shown in order of production process, for explaining the technique of forming a wiring pattern disclosed in Patent document 1. First, an ink 31 containing Ag and Pd nanoparticles is applied on a substrate 1 by a spin coating. The ink 31 applied is then dried to form a thin film 32. Next, as shown in FIG. 1B, an etching process is performed on the thin film 32 in such a way as to only leave necessary portions intact. Thus, a photoresist 2 with an wiring pattern is formed on the thin film 32. Next, as shown in FIG. 1C, using the photoresist 2 as a mask, unnecessary portions of the thin film 32 is removed by etching. And then, as shown in FIG. 1D, the photoresist 2 on the thin film 32 is removed. Subsequently, when the substrate 1 is baked in a furnace as shown in FIG. 1E, the thin film 32 is baked, and a wiring 33 is formed which is composed of a thin film made of an alloy of Ag and Pd.
In the above-described method of forming a wiring pattern disclosed in Patent document 1, an ink material is applied on the entire surface of the substrate, and therefore the manufacturing cost is increased.
Hence, in order to achieve reduction in the manufacturing cost by reducing the amount of nanoparticles-containing inks to be used, an inkjet system is employed that enables local application of inks. However, this inkjet system has poor accuracy in positioning of inks, and is not generally suited for the formation of fine patterns.
In order to solve the conventional problems, Japanese Patent Laid-Open No. 2003-188497 (hereinafter referred to as Patent document 2) discloses a technique in which a water repellent film is formed.
The technique disclosed in Patent Document 2 involves: after forming a water repellent film on a substrate, removing the water repellent film formed on the wiring forming regions by irradiating it with laser beams; and applying, using an ink jet system, a nanoparticles-containing ink onto regions where the water repellent film has been removed.
FIGS. 2A to 2E show cross-sectional views of the principal portions of the substrate, which are shown in order of production process, for explaining the technique of forming a wiring pattern disclosed in Patent document 2. First, a water repellent film 4 such as fluorine resin is formed on the surface of the substrate 1, as shown in FIG. 2A. Next, as shown in FIG. 2B, a laser beam 5 is irradiated on the surface of the substrate 1 in accordance with an wiring pattern to be formed. As a consequence, the portions of the water repellent film 4, irradiated with the laser beam 5, are removed. Thus, the surface of the substrate 1 is exposed. Next, as shown in FIG. 2C, when the exposed surface is roughened, the surface of the substrate 1 is cleaned to remove scrap pieces of the substrate 1, followed by drying of the surface of the substrate 1. Next, as shown in FIG. 2D, using an inkjet system, a jet of the nanoparticles-containing ink 31 is directed at regions where the water repellent film 4 has been removed. In the drawing process in which the ink 31 is injected, an inkjet apparatus equipped with inkjet nozzles is used to direct a jet of the nanoparticles-containing ink at the surface of the substrate 1, and thereby a conductive circuit pattern is formed. The conductive circuit pattern is drawn by driving the XY table according to the wiring pattern to be formed on the substrate 1. Even when the nanoparticles-containing ink 31 lies off the regions where the water repellent film 4 has been removed, the hydrophobic properties of the water repellent film 4 allows the nanoparticles-containing ink 31 to move to the wiring regions. While maintaining this state, the formed pattern is dried. Thereafter, as shown in FIG. 2E, the water repellent film 4 on the substrate 1 is removed by cleaning, and then the substrate 1 is dried. Subsequently, the ink 31 formed on the wiring region is subjected to a baking treatment, whereby a wiring pattern is formed.
In patent document 1 described above, after applying the nanoparticles-containing ink onto the substrate by spin coating, a photolithography process and an etching process are adopted for the formation of a wiring pattern.
For this reason, consumption of materials may be increased when the ink is applied by spin coating, and the ink may be applied on the unnecessary portions of a substrate. Thus, materials are not efficiently used and the manufacturing cost is increased.
In such cases, inkjet systems can be used to apply the nanoparticles-containing ink onto only wiring portions, and thus materials can be used more efficiently. However, conventional inkjet systems have poor accuracy in positioning of inks, and are not suited for the formation of fine wiring patterns. Moreover, in Patent document 2, a water repellent film is formed on the surface of the substrate to form a finer pattern by using an inkjet system in order to relax the accuracy in positioning of the ink. However, in Patent document 2, a laser beam is directly irradiated onto the water repellent film to form a pattern. Accordingly, there is a problem that the shape and thickness of the water repellent film that needs to be removed cannot be controlled accurately. As a result, the shape of a wiring pattern, reflecting the portions of the water repellent film that have been removed, cannot be controlled accurately.
In addition, when multilayered wiring patterns are to be formed, a wiring pattern in the lower layers are damaged by the laser beam, and voids are generated in insulating films provided between the upper and lower layers of a wiring pattern.